Oral daily administration of aspirin or indomethacin retarded growth of experimental tumors in mouse. Aspirin treatment, 150mg/kg twice daily, inhibited growth of a transplantable mast-cell ascites tumor (P815) by 39-43 per cent (p less than 0.001) and of a subcutaneously transplanted Lewis lung carcinoma by 52 per cent (p less than 0.025) without adversely affecting body growth. The total serotonin, histamine and histidine decarboxylase content of the ascites tumor was also reduced as was the urinary excretion of the amines. Treatment with 3 and 5 mg/kg indomethacin resulted in 40 (p less than 0.01) and 80 (p less than 0.001) per cent reduction, respectively, in ascites tumor growth. With the higher dose of indomethacin, no tumor was observed in half of the animals inoculated with tumor, although signs of indomethacin toxicity (reduced body growth, gastric lesion) was evident in the animals.